This chapter describes the large-displacement indentation test method for examining elastic-plastic behaviors of vertically aligned carbon nanotube arrays (VA-CNTs). The principle of this test is explained by using a cavity expansion model. The experiments have been performed on VA-CNTs synthesized by the chemical vapor deposition (CVD) method. Under a cylindrical, flat indenter, the VA-CNTs exhibit two distinct deformation stages: a short, elastic deformation at small displacement and a plateau-like, plastic deformation at large displacement. The critical indentation stress, a measure of yield stress or collapsing stress of the VA-CNT arrays, has been obtained. The deformation mechanism of the VA-CNTs at large displacement is revealed with scanning electronic microscope (SEM) images of the deformed VA-CNTs and finite element simulations.